I. Field of the Invention
The present invention relates to a color TV signal conversion device used for converting an NTSC color TV signal to a PAL color TV signal in a video disc reproduction system or a video tape recorder.
The NTSC color television signal includes a luminance signal and a carrier chrominance signal. The carrier chrominance signal has two color difference signal components R-Y and B-Y which modulate a color subcarrier in accordance with quadrature modulation. In the case of recording the color TV signal on a recording medium such as a video disc, the carrier chrominance signal is frequency-converted to a low frequency and is recorded on the recording medium. Therefore, when a pickup signal from the video disc is reproduced, the low-frequency carrier chrominance signal is demodulated to an original frequency carrier chrominance signal.
FIG. 1 shows a video disc reproduction system for reproducing the signals recorded by the NTSC system.
A color TV signal to be detected and reproduced from the video disc is separated into a low-frequency carrier chrominance signal Ch, and a luminance signal (Y signal) which includes a synchronizing signal. The signal Ch and the Y signal are supplied to input terminals 11 and 12, respectively.
The carrier frequency of the carrier chrominance signal Ch applied to the input terminal 11 is interleaved with that of the Y signal, for example, in the following manner: EQU 195/2.fH=1.534091 (MHz)
where fH is the line frequency.
In order to reconvert the 1.53-MHz chrominance signal (low-frequency converted chrominance signal) to the 3.58-MHz chrominance signal (original frequency carrier chrominance signal), the 1.53-MHz chrominance signal is supplied to a frequency converter 13. The frequency converter 13 multiplies the 1.53-MHz chrominance signal and a carrier wave (CW) signal (5.11 MHz=1.53 MHz+3.58 MHz). The 5.11-MHz carrier wave signal is produced by a VCO (voltage controlled oscillator) 14. The multiplied signal is supplied to a band-pass filter 15 having a 3.58 MHz band-pass characteristic. The NTSC 3.58-MHz chrominance signal is then obtained from the band-pass filter 15.
This 3.58-MHz chrominance signal is mixed by a mixer 16 with the Y signal applied to the input terminal 12. An NTSC color TV signal then appears at an output terminal 17.
A video disc player is subject to a time base error of signal components which is caused by wow and flutter of a turntable or the like. Such a time base error causes significant jitter in the 1.53-MHz chrominance signal. A video disc player generally has an automatic phase control (APC) loop in order to eliminate such a problem. More specifically, the chrominance signal reconverted to a frequency of 3.58 MHz is supplied to a phase comparator 18 which compares the phase of this input signal with that of a reference signal having a frequency of 3.58 MHz during each burst period. The 3.58-MHz reference signal is produced by a 3.58-MHz oscillator 19. A burst gate pulse is obtained from a burst gate pulse generator 21, which receives an output signal from a sync separator 20 which separates a horizontal synchronizing signal from the Y signal supplied to the input terminal 12, and which then supplies an output signal to the phase comparator 18. An output signal from the phase comparator 18 is held for a 1-H period by a sample and hold circuit 22, and is supplied to an oscillation frequency control end of the VCO 14 through a low-pass filter 23.
A PAL color TV system has, in addition to the luminance signal, two chrominance signal components (color difference signals) according to which the subcarrier is quadrature-modulated. One (R-Y signal) of the two chrominance signal components is inverted for each successive line. A carrier frequency fsc of the PAL system is generally selected to be 4.43 MHz.
FIG. 2 shows an example of a demodulation circuit for demodulating such a
color TV signal. Referring to FIG. 2, a PAL composite signal is supplied to an input terminal 25 and is then supplied to a C-Y separator 26 which produces a Y signal onto a line 27 and a carrier chrominance signal onto a line 28. The carrier chrominance signal is subjected to addition by an adder 30 and subtraction by a subtractor 31 of a delayed carrier chrominance signal from a 1-H delay line 29. A B-Y signal appears at the output end of the adder 30, while an R-Y signal appears at the output end of the subtractor 31. These B-Y and R-Y signals are respectively supplied to B-Y and R-Y demodulators 32 and 33. The line 28 is also connected to a local subcarrier oscillator 35 through a burst phase discriminator 34. A reference subcarrier signal of 4.43 MHz is supplied to the B-Y demodulator 32 through a 90.degree. phase shifter 36 and is also supplied to the R-Y demodulator 33 through a line changeover switch 37. Since the R-Y signal changes its polarity for each successive line, the reference subcarrier signal to be supplied to the R-Y demodulator 33 must be inverted by 180.degree. for each successive line. Thus, the line changeover switch 37 has a 180.degree. phase shifter 371 and a changeover switch 372. The changeover switch 372 is switched between a 180.degree. phase shift line 381 and a direct coupling line 382 for each line.
In order to control the switching operation of the switch 372, a sync signal is separated from the PAL composite signal at the input terminal 25 through a sync separator 39. The sync signal is then supplied to a flip-flop 40 which produces a signal which is used as a line changeover signal.
The signals sync-detected by the B-Y and R-Y demodulators 32 and 33 are supplied to a matrix circuit 41 which produces blue, green and red signals B, G and R, respectively.
When the signal recorded by the NTSC system on the video disc is reproduced by an NTSC reproduction system, the reproduced color TV signal is of the NTSC system. Therefore, even if the NTSC reproduced color TV signal is supplied to a color TV receiver of the PAL system, color reproduction images cannot be obtained.
When the NTSC reproduced color TV signal is supplied to a PAL color TV receiver, a marginaly satisfactory reproduced image can be obtained wherein the luminance signal seems unnaturally bright because of different standards. However, as far as the chrominance signal is concerned, since different signal processing systems are adopted in the NTSC and PAL systems, a good reproduced color image cannot be obtained.
In European countries where the PAL system is adopted, the need arises to reproduce the NTSC color TV signal with a PAL color TV receiver. In order to respond to such a demand, in the present satellite relay system, the NTSC color TV signal is completely demodulated by the NTSC system and the demodulated signal is then converted to a PAL color TV signal.
However, it is impossible in practice to apply the techniques used in the satellite relay system to a video disc player whose circuit scale, cost and size are limited.